U.S. patent application number 13/388082 was filed with the patent office on 2012-05-31 for grain oriented electrical steel sheet.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Takeshi Imamura, Mineo Muraki, Yukihiro Shingaki.
Application Number | 20120131982 13/388082 |
Document ID | / |
Family ID | 43529499 |
Filed Date | 2012-05-31 |
United States Patent
Application |
20120131982 |
Kind Code |
A1 |
Imamura; Takeshi ; et
al. |
May 31, 2012 |
GRAIN ORIENTED ELECTRICAL STEEL SHEET
Abstract
An electrical steel sheet contains, as components, by mass %,
0.005% or less of C, 1.0% to 8.0% of Si, and 0.005% to 1.0% of Mn;
one or more selected from Nb, Ta, V, and Zr such that a total
content thereof is 10 to 50 ppm; and the balance being Fe and
unavoidable impurities, wherein at least 10% of the content of Nb,
Ta, V, and Zr is in the form of precipitates; the precipitates have
an average diameter (equivalent circle diameter) of 0.02 to 3
.mu.m; and secondary recrystallized grains of the steel sheet have
an average grain size of 5 mm or more.
Inventors: |
Imamura; Takeshi;
(Kurashiki, JP) ; Shingaki; Yukihiro; (Kurashiki,
JP) ; Muraki; Mineo; (Chiba, JP) |
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
43529499 |
Appl. No.: |
13/388082 |
Filed: |
July 30, 2010 |
PCT Filed: |
July 30, 2010 |
PCT NO: |
PCT/JP2010/063343 |
371 Date: |
February 13, 2012 |
Current U.S.
Class: |
72/362 ;
148/328 |
Current CPC
Class: |
C22C 38/06 20130101;
C21D 8/1272 20130101; C22C 38/20 20130101; C21D 8/1283 20130101;
C22C 38/002 20130101; H01F 1/16 20130101; C22C 38/02 20130101; C22C
38/34 20130101; C22C 38/001 20130101; C22C 38/04 20130101; C21D
9/46 20130101; C22C 38/12 20130101; C21D 2201/05 20130101; C22C
38/008 20130101; C21D 8/12 20130101 |
Class at
Publication: |
72/362 ;
148/328 |
International
Class: |
B21D 31/00 20060101
B21D031/00; C22C 38/04 20060101 C22C038/04; C22C 38/16 20060101
C22C038/16; C22C 38/12 20060101 C22C038/12; C22C 38/34 20060101
C22C038/34; C22C 38/08 20060101 C22C038/08; C22C 38/60 20060101
C22C038/60; C22C 38/02 20060101 C22C038/02; C22C 38/18 20060101
C22C038/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2009 |
JP |
2009-179494 |
Claims
1. A grain oriented electrical steel sheet comprising, by mass %,
0.005% or less of C, 1.0% to 8.0% of Si, and 0.005% to 1.0% of Mn;
one or more selected from the group consisting of Nb, Ta, V, and Zr
such that a total content thereof is 10 to 50 ppm; and the balance
being Fe and unavoidable impurities, wherein at least 10% of the
content of Nb, Ta, V, and Zr is in the form of precipitates; the
precipitates having an average diameter (equivalent circle
diameter) of 0.02 to 3 .mu.m; and secondary recrystallized grains
of the steel sheet have an average grain size of 5 mm or more.
2. The grain oriented electrical steel sheet according to claim 1,
further comprising at least one selected from the group consisting
of, by mass %, 0.010% to 1.50% of Ni, 0.01% to 0.50% of Cr, 0.01%
to 0.50% of Cu, 0.005% to 0.50% of P, 0.005% to 0.50% of Sn, 0.005%
to 0.50% of Sb, 0.005% to 0.50% of Bi, and 0.005% to 0.100% of
Mo.
3. The grain oriented electrical steel sheet according to claim 1,
wherein a groove is formed in a surface of the steel sheet, the
groove having a shape of a solid line or broken lines, a width of
50 to 1,000 .mu.m, and a depth of 10 to 50 .mu.m, and extending at
an angle of 15.degree. or less with respect to a direction
perpendicular to a rolling direction of the steel sheet.
4. A method for producing an iron core, comprising shearing the
grain oriented electrical steel sheet according to claim 1 to
provide sheets and subsequently stacking the sheets without
subjecting the sheets to stress relief annealing.
5. The grain oriented electrical steel sheet according to claim 2,
wherein a groove is formed in a surface of the steel sheet, the
groove having a shape of a solid line or broken lines, a width of
50 to 1,000 .mu.m, and a depth of 10 to 50 .mu.m, and extending at
an angle of 15.degree. or less with respect to a direction
perpendicular to a rolling direction of the steel sheet.
6. A method for producing an iron core, comprising shearing the
grain oriented electrical steel sheet according to claim 2 to
provide sheets and subsequently stacking the sheets without
subjecting the sheets to stress relief annealing.
7. A method for producing an iron core, comprising shearing the
grain oriented electrical steel sheet according to claim 3 to
provide sheets and subsequently stacking the sheets without
subjecting the sheets to stress relief annealing.
8. A method for producing an iron core, comprising shearing the
grain oriented electrical steel sheet according to claim 5 to
provide sheets and subsequently stacking the sheets without
subjecting the sheets to stress relief annealing.
Description
RELATED APPLICATIONS
[0001] This is a .sctn.371 of International Application No.
PCT/JP2010/063343, with an international filing date of Jul. 30,
2010 (WO 2011/013858 A1, published Feb. 3, 2011), which is based on
Japanese Patent Application No. 2009-179494, filed Jul. 31, 2009,
the subject matter of which is incorporated by reference.
TECHNICAL FIELD
[0002] This disclosure relates to a grain oriented electrical steel
sheet suitably used as, for example, an iron-core material for
transformers and, in particular, reduces degradation of magnetic
characteristics in the case of the sheet being sheared.
BACKGROUND
[0003] Electrical steel sheets are a material widely used for iron
cores of various transformers, motors, and the like. In particular,
among them, electrical steel sheets that are referred to as grain
oriented electrical steel sheets have crystal grains that are
highly oriented in {110}<001>, which is referred to as the
Goss orientation.
[0004] In the production of such grain oriented electrical steel
sheets, a technique of causing secondary recrystallization of
crystal grains having the Goss orientation to occur during final
annealing with precipitates referred to as an inhibitor is
generally used.
[0005] For example, Japanese Examined Patent Application
Publication No. 40-15644 discloses a method of making Al and S
serving as inhibitor-forming elements be present in predetermined
amounts, that is, a method of using AlN and MnS as inhibitors.
Japanese Examined Patent Application Publication No. 51-13469
discloses a method of making at least one of S and Se be present in
a predetermined amount, that is, a method of using MnS or MnSe as
an inhibitor. These methods are industrially used. Furthermore, as
proposed in Japanese Unexamined Patent Application Publication No.
2000-129356, a technique of developing Goss oriented grains by the
action of secondary recrystallization even in steel sheets having
no inhibitor-forming elements has been recently presented.
[0006] In the technique described in JP '356, by minimizing
impurities such as inhibitor-forming elements, the grain boundary
misorientation dependency of grain boundary energy of grain
boundary in the occurrence of primary recrystallization is elicited
so that Goss oriented grains are developed by secondary
recrystallization without inhibitors.
[0007] Since this method does not require inhibitor-forming
elements, the necessity of the step of purifying to remove
inhibitor-forming elements is eliminated. In addition, it is not
necessary to perform purification annealing at a high temperature
and a step of finely dispersing inhibitor-forming elements in steel
is no longer necessary. Hence, slab reheating at a high temperature
that was indispensable for the fine dispersion is also no longer
necessary. Thus, the method is highly advantageous in terms of
steps, cost, and maintenance of equipment and the like.
[0008] Among various characteristics of grain oriented electrical
steel sheets, an iron loss characteristic directly relates to
energy loss of products and is considered to be the most important
characteristic. To improve the iron loss characteristic, it is
believed that a value represented by W.sub.17/50 (energy loss at an
excitation magnetic flux density of 1.7 T and an excitation
frequency of 50 Hz) should be decreased.
[0009] In transformers for which grain oriented electrical steel
sheets are used, the iron loss characteristic is also considered as
an important characteristic. Even after transformers are produced,
the transformers that are used need to be periodically measured in
terms of iron loss characteristic for the purpose of controlling
the iron loss characteristic.
[0010] In general, electrical steel sheet products have the shape
of a sheet and are cut to have a predetermined size in the
production of transformers. This cutting is generally performed by
shearing (also referred to as slit processing) in which two blades
vertically press against each other (the blades finally slide over
each other) as in a pair of scissors.
[0011] In the thus-sheared steel sheets, the processed surfaces are
formed by tearing due to a shearing force and a large amount of
strain is introduced into the steel sheets. Accordingly,
degradation of magnetic characteristics due to the introduced
strain tends to occur in sheared electrical steel sheets, which is
problematic.
[0012] As a method of reducing degradation of magnetic
characteristics due to shearing, stress relief annealing of
annealing at 700.degree. C. to 900.degree. C. for several hours may
be performed after shearing. However, stress relief annealing is
performed only for small transformers having a size (length) of 500
mm or less and it cannot be performed for, for example, iron cores
of large transformers having a size of several meters.
[0013] Accordingly, a technique has been demanded that can reduce
degradation of magnetic characteristics due to shearing in
electrical steel sheets for large transformers having a size of
several meters.
SUMMARY
[0014] We provide: [0015] 1. A grain oriented electrical steel
sheet characterized by comprising, by mass %, 0.005% or less of C,
1.0% to 8.0% of Si, and 0.005% to 1.0% of Mn; one or more selected
from Nb, Ta, V, and Zr such that a total content thereof is 10 to
50 ppm; and the balance being Fe and unavoidable impurities,
wherein at least 10% of the content of Nb, Ta, V, and Zr is in the
form of precipitates; the precipitates have an average diameter
(equivalent circle diameter) of 0.02 to 3 .mu.m; and secondary
recrystallized grains of the steel sheet have an average grain size
of 5 mm or more. [0016] 2. The grain oriented electrical steel
sheet according to 1 above, characterized by further comprising at
least one selected from, by mass %, 0.010% to 1.50% of Ni, 0.01% to
0.50% of Cr, 0.01% to 0.50% of Cu, 0.005% to 0.50% of P, 0.005% to
0.50% of Sn, 0.005% to 0.50% of Sb, 0.005% to 0.50% of Bi, and
0.005% to 0.100% of Mo. [0017] 3. The grain oriented electrical
steel sheet according to 1 or 2 above, characterized in that a
groove is formed in a surface of the steel sheet, the groove having
a shape of a solid line or a broken line, a width of 50 to 1,000
.mu.m, and a depth of 10 to 50 .mu.m, and extending at an angle of
15.degree. or less with respect to a direction perpendicular to a
rolling direction of the steel sheet. [0018] 4. A method for
producing an iron core, characterized by shearing the grain
oriented electrical steel sheet according to any one of 1 to 3
above to provide sheets and subsequently stacking the sheets
without subjecting the sheets to stress relief annealing.
[0019] Degradation of magnetic characteristics of grain oriented
electrical steel sheets due to shearing can thus be effectively
suppressed and iron cores having less energy loss can be produced
for transformers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 illustrates the relationship between Nb content in
steel (abscissa axis: ppm) and the amount of degradation of iron
loss due to shearing (.DELTA.W) (ordinate axis: W/kg).
[0021] FIG. 2 illustrates the relationship between the crystal
grain size of secondary recrystallized grains (abscissa axis: mm)
and the amount of degradation of iron loss due to shearing
(.DELTA.W) (ordinate axis: W/kg).
DETAILED DESCRIPTION
[0022] Hereinafter, our steel sheets and methods will be
specifically described.
[0023] The reasons why the component composition of a steel sheet
is limited to the above-described ranges will be first described.
Note that "%" and "ppm" for components of a steel sheet
respectively represent mass % and mass ppm, unless otherwise
stated.
C: 0.005% or less
[0024] C is an element that unavoidably enters steel. Since C
causes degradation of magnetic characteristics by magnetic aging,
the C content is desirably minimized. However, it is difficult to
completely remove C and a C content of 0.005% or less is allowable
in view of production cost, preferably 0.002% or less. There is no
reason for particularly defining the lower limit of the C content.
The C content is industrially more than zero.
Si: 1.0% to 8.0%
[0025] Si is an element necessary to increase the resistivity of
steel and achieve improvement in terms of iron loss in final
product sheets. When the Si content is less than 1.0%, such an
advantage is not sufficiently provided. When the Si content is more
than 8.0%, the saturation flux density of a steel sheet
considerably decreases. Accordingly, the Si content is limited to a
range of 1.0% to 8.0%. The lower limit of the Si content is
preferably 3.0%. The upper limit of the Si content is preferably
3.5%.
Mn: 0.005% to 1.0%
[0026] Mn is an element necessary to enhance formability in hot
rolling. When the Mn content is less than 0.005%, the effect of
enhancing workability is not sufficiently provided. When the Mn
content is more than 1.0%, secondary recrystallization becomes
unstable and magnetic characteristics are degraded. Accordingly,
the Mn content is limited to a range of 0.005% to 1.0%. The lower
limit of the Mn content is preferably 0.02%. The upper limit of the
Mn content is preferably 0.20%.
[0027] It is necessary to make one or more selected from Nb, Ta, V,
and Zr (hereafter, referred to as "Nb or the like") be contained as
precipitate-forming elements such that the total content thereof is
10 to 50 ppm. This is because, when the total content of Nb or the
like is less than 10 ppm, precipitates that improve iron loss,
which are a main feature, are not sufficiently generated. When the
total content of Nb or the like is more than 50 ppm, the iron loss
characteristic of a material itself is degraded as described above.
Thus, the upper limit of the total content is defined as 50 ppm.
The total content is preferably in the range of 10 to 30 ppm.
[0028] It is necessary that the precipitates of Nb or the like are
present in a percentage of 10% or more and the precipitates have an
average diameter (equivalent circle diameter) of 0.02 to 3 .mu.m.
When the average diameter is less than 0.02 .mu.m, the precipitates
are too small and stress is less likely to be concentrated. When
the average diameter is more than 3 .mu.m, the frequency of the
presence (number) of the precipitates becomes small and the number
of portions where stress is concentrated becomes small. The
precipitates preferably have an average diameter of 0.05 to 3
.mu.m. The lower limit is more preferably 0.12 .mu.m, still more
preferably 0.33 .mu.m. The upper limit is more preferably 1.2
.mu.m, still more preferably 0.78 .mu.m.
[0029] The precipitation percentage of the precipitates of Nb or
the like is preferably 20% or more, more preferably 31% or more,
still more preferably 48% or more. It is not necessary to define
the upper limit and a precipitation percentage of 100% does not
cause problems.
[0030] The average diameter of the precipitates of Nb or the like
is preferably determined in the following manner: a section of an
obtained sample is observed with a scanning electron microscope;
micrographs of about 10 fields of view are taken at a magnification
of about 10,000; the micrographs are subjected to image analysis
and the average of equivalent circle diameters is determined. The
percentage of precipitates (precipitation percentage) is preferably
measured in accordance with the method described in Experiment 1
below. When a steel sheet contains two or more elements as Nb or
the like, the total content (mass %) of Nb or the like in
precipitates should be divided by the total content (mass %) of Nb
or the like in the steel sheet.
[0031] As a precipitate-forming element, one or more selected from
Nb, V, and Zr are preferred because they are less likely to form
defects in steel sheets during hot rolling. In particular, Nb is
preferred because defects during hot rolling can be reduced. In
such cases, the essential range is also 10 to 50 ppm and the
preferred range is also 10 to 30 ppm and a preferred diameter of
precipitates and a preferred precipitation percentage are the same
as those described above.
[0032] To adjust the diameter and the precipitation percentage of
precipitates of Nb or the like, it is effective to control, in
purification annealing, the maximum steel sheet temperature and a
cooling rate in the subsequent cooling from 900.degree. C. to
500.degree. C. This is because such precipitates can be controlled
in terms of diameter and precipitation percentage by performing
purification annealing at a high temperature to dissolve the
precipitates and performing cooling to cause reprecipitation.
[0033] In such a phenomenon, as in general precipitation phenomena,
a high cooling rate results in a low amount of precipitates (a
portion remains in the form of a solid solution) and a small
diameter of precipitates and, in contrast, a low cooling rate tends
to result in the reverse state.
[0034] As described above, to exhibit the effect of decreasing
.DELTA.W by addition of a precipitate-forming element, it is
necessary that the average grain size of secondary recrystallized
grains of a material is 5 mm or more. Although such a grain size is
a general grain size in electrical steel sheets for large
transformers having a size of several meters, regardless of such a
sheet size, by controlling a temperature increase rate and an
atmosphere in secondary recrystallization, the average grain size
can be controlled to be 5 mm or more. The average grain size of
secondary recrystallized grains is preferably determined by the
method described in Experiment 2 below.
[0035] Note that a method of decreasing .DELTA.W by making the
average grain size of secondary recrystallized grains be less than
5 mm is not preferred because the absolute values of iron loss and
magnetic flux density become poor.
[0036] The basic component composition and the like have been
described so far.
[0037] If necessary, elements described below may appropriately be
contained.
Ni: 0.010% to 1.50%
[0038] To enhance magnetic characteristics, Ni may be added. In
such a case, when the amount of Ni added is less than 0.010%,
magnetic characteristics are not sufficiently enhanced. When the
amount of Ni added is more than 1.50%, secondary recrystallization
becomes unstable and magnetic characteristics may be degraded.
Accordingly, the Ni content is preferably made in the range of
0.010% to 1.50%.
Cr: 0.01% to 0.50%, Cu: 0.01% to 0.50%, P: 0.005% to 0.50%
[0039] To decrease iron loss, at least one of Cr, Cu, and P may be
added.
[0040] However, when the amounts of the elements added are less
than the lower limits, the effect of decreasing iron loss is not
sufficiently provided. When the amounts of the elements added are
more than the upper limits, development of secondary recrystallized
grains is suppressed, resulting in an unintended increase in iron
loss. Accordingly, the contents of the elements are preferably in
the ranges described above, respectively.
Sn: 0.005% to 0.50%, Sb: 0.005% to 0.50%, Bi: 0.005% to 0.50%, Mo:
0.005% to 0.100%
[0041] To increase magnetic flux density, at least one of Sn, Sb,
Bi, and Mo may be added.
[0042] However, when the amounts of the elements added are less
than the lower limits, the effect of enhancing the magnetic
characteristic is not sufficiently provided. When the amounts of
the elements added are more than the upper limits, development of
secondary recrystallized grains is suppressed, resulting in
degradation of the magnetic characteristic. Accordingly, the
contents of the elements are preferably in the ranges described
above, respectively.
[0043] In summary, an electrical steel sheet may further contain at
least one selected from 0.010% to 1.50% of Ni, 0.01% to 0.50% of
Cr, 0.01% to 0.50% of Cu, 0.005% to 0.50% of P, 0.005% to 0.50% of
Sn, 0.005% to 0.50% of Sb, 0.005% to 0.50% of Bi, and 0.005% to
0.100% of Mo. Further, as for a subset constituted by elements
freely selected from the group of these elements, at least one
selected from elements (group) constituting the subset may be made
to be contained.
[0044] In addition, if necessary, at least one combination of
inhibitor-forming elements (for example, AlN-forming elements: Al
and N, MnS-forming elements: Mn and S, MnSe-forming elements: Mn
and Se, and TiN-forming elements: Ti and N) in a necessary amount
(publicly known) can be contained.
[0045] The balance is Fe and normal unavoidable impurities.
Examples of the unavoidable impurities include P, S, O, Al, N, Ti,
Ca, and B (when Al and the like are not added as inhibitor-forming
elements, they are impurities).
[0046] Grooves are preferably formed in a surface of a steel sheet,
the grooves having the shape of a solid line or broken lines, a
width of 50 to 1,000 .mu.m, and a depth of 10 to 50 .mu.m, and
extending in a direction to intersect at an angle of 15.degree. or
less in a direction perpendicular to the rolling direction. The
formation of such grooves provides the magnetic domain refining
effect, resulting in a further decrease in iron loss. The space
between the grooves (pitch) is preferably about 2 to 7 mm. When
grooves extend at an angle of 0.degree. with respect to a direction
perpendicular to a rolling direction, in a strict sense, the
grooves do not intersect the direction perpendicular to the rolling
direction. However, such a case is also referred to as an
"intersection." In summary, grooves should be formed at an angle of
15.degree. or less with respect to a direction perpendicular to a
rolling direction.
[0047] As a result of the formation of such grooves, the iron loss
of an electrical steel sheet decreases by about 0.17 W/kg. Such an
advantage was found to be achieved regardless of selection of an
element from Nb, Ta, V, and Zr.
[0048] Hereinafter, a preferred method for producing a grain
oriented electrical steel sheet will be described. As main steps in
this production method, production steps for a standard grain
oriented electrical steel sheet can be used. Specifically, a series
of steps can be used in which slabs produced from a molten steel
adjusted to have a predetermined component composition are
hot-rolled; the resultant hot-rolled sheets are optionally
subjected to hot-rolled sheet annealing and then subjected to a
single cold-rolling step or two or more cold-rolling steps that
include an intermediate annealing therebetween to have a final
sheet thickness. The steel sheets are subsequently subjected to
recrystallization annealing, then to purification annealing, and
optionally to flattening annealing and the steel sheets are then
coated.
[0049] In the case of adjusting the component composition of molten
steel, when the amount of C added is more than 0.10%, it is
difficult to decrease in subsequent steps the C content to 50 ppm
(0.005%) or less, which does not cause magnetic aging. Accordingly,
the amount of C added in molten steel is preferably 0.10% or
less.
[0050] The Si content may be adjusted to be 1.0% to 8.0%, which is
the finally required content, in the adjustment of the component
composition of molten steel. When a method of increasing Si content
by siliconization or the like is employed in a step after the
production of slabs, the amount of Si added to molten steel may be
less than the finally required content.
[0051] It is difficult to add or remove Nb, Ta, V, and Zr, which
are essential components, during steps after the molten steel
state. Accordingly, it is most desirable that a required amount of
such a component be added in the adjustment of the component
composition of molten steel.
[0052] From molten steel containing the components described above,
slabs may be produced by a standard ingot making process or a
standard continuous casting process, or otherwise thin cast slabs
having a thickness of 100 mm or less may be produced by direct
casting process. Although slabs are heated and hot-rolled in a
standard manner, slabs after being cast may be instead directly
hot-rolled without being heated. In the case of thin cast slabs, it
may be hot-rolled or jump straight to next steps without being
hot-rolled.
[0053] The heating temperature of slabs to be hot-rolled in a
component system containing an inhibitor-forming element is
normally a high temperature of about 1,400.degree. C. In contrast,
the heating temperature in a component system without
inhibitor-forming elements is normally a low temperature of
1,250.degree. C. or less, which is advantageous in terms of
cost.
[0054] If necessary, hot-rolled sheet annealing is then performed.
To achieve good magnetic properties, the temperature of the
hot-rolled sheet annealing is preferably 800.degree. C. or more and
1,150.degree. C. or less. This is because, when the temperature of
the hot-rolled sheet annealing is less than 800.degree. C., a band
texture due to hot rolling remains and it becomes difficult to
achieve a primary recrystallization texture having uniformly-sized
grains. Accordingly, the hot-rolled sheet annealing provides a
relatively limited effect of promoting development of secondary
recrystallized grains. When the temperature of the hot-rolled sheet
annealing is more than 1,150.degree. C., crystal grains after the
hot-rolled sheet annealing become coarse. Accordingly, also in this
case, it becomes difficult to achieve a primary recrystallization
texture having uniformly-sized grains.
[0055] After the hot-rolled sheet annealing, one or more
cold-rolling steps optionally including a process an intermediate
annealing therebetween are performed and recrystallization
annealing is then performed. To further enhance magnetic
characteristics, it is effective to perform cold rolling in a
temperature range of 100.degree. C. to 300.degree. C. and/or to
perform one or more aging treatments in a range of 100.degree. C.
to 300.degree. C. during the cold rolling process. In the case of
performing recrystallization annealing, when decaburization is
necessary, a wet atmosphere is employed in the recrystallization
annealing. However, when decaburization is not necessary, the
recrystallization annealing may be performed in a dry atmosphere.
After the recrystallization annealing, a technique of increasing Si
content by siliconization may be further employed.
[0056] When iron loss is considered as an important factor and a
forsterite coating is subsequently formed, the sheets are coated
with an annealing separator mainly containing MgO and then
subjected to final annealing (purification annealing) to develop a
secondary recrystallization texture and to form a forsterite
coating.
[0057] When a blanking property is considered as an important
factor and a forsterite coating is not intentionally formed, an
annealing separator is not applied or, even when an annealing
separator is applied, silica, alumina, or the like should be used
instead of MgO forming a forsterite coating.
[0058] When such an annealing separator is applied, for example,
electrostatic coating without involving water content is
effectively performed. A heat-resistant inorganic material sheet
(silica, alumina, or mica) may be used.
[0059] The final annealing is sufficiently performed at a
temperature allowing for secondary recrystallization, and desirably
at 800.degree. C. or more. An annealing condition under which
secondary recrystallization is completed is desirable and it is
generally desirable that the sheets be held at a temperature of
800.degree. C. or more for 20 or more hours. When a blanking
property is considered as an important factor and a forsterite
coating is not formed since secondary recrystallization only needs
to be completed, the holding temperature is desirably about
850.degree. C. to 950.degree. C. and the final annealing may be
finished with this holding treatment. When iron loss is considered
as an important factor or the noise of a transformer is to be
reduced, and a forsterite coating is formed, the temperature is
advantageously increased to about 1,200.degree. C.
[0060] In cooling in such a high-temperature annealing, the cooling
is desirably performed at a rate of 5.degree. C./hr to 100.degree.
C./hr at least in a temperature range of 900.degree. C. to
500.degree. C. When cooling is performed from a holding temperature
less than 900.degree. C., the cooling is desirably performed at a
rate of 5.degree. C./hr to 100.degree. C./hr in a temperature range
of the holding temperature to 500.degree. C. This is because, when
the cooling rate is more than 100.degree. C./hr in such a
temperature range, there may be cases where precipitates become
excessively fine or precipitation from a solid solution does not
occur. When the cooling rate is less than 5.degree. C./hr, there
may be cases where the diameter of precipitates becomes excessively
large, or the cooling time becomes excessively long resulting in,
for example, degradation of productivity. The lower limit of the
cooling rate is more preferably 7.8.degree. C./hr. The upper limit
of the cooling rate is more preferably 30.degree. C./hr. In view of
achieving results with stability, the upper limit of the cooling
rate is still more preferably 14.degree. C./hr.
[0061] After the final annealing, to remove an annealing separator
adhering, it is effective to perform cleaning with water, brushing,
and/or pickling. After that, it is effective to subject the sheets
to flattening annealing to correct the shape thereof for the
purpose of decreasing iron loss.
[0062] When the steel sheets are laminated and used to achieve
improvement in terms of iron loss, it is effective to form
insulation coatings on the surfaces of the steel sheets before or
after the flattening annealing. To decrease iron loss, coatings
that can impart tension to steel sheets are desirable. When a
method of coating the surfaces of a steel sheet with an inorganic
substance by a tension coating application method with a binder, a
physical vapor deposition method, a chemical vapor deposition
method, or the like is employed, the coating films exhibit high
adhesion and iron loss is considerably decreased, which is
particularly desirable.
[0063] To decrease iron loss, a magnetic domain refining treatment
is desirably performed. An example of this treatment is, as
generally performed, a method of forming grooves in final product
sheets or linearly introducing thermal strain or impact strain with
laser or plasma into final product sheets, or a method of forming
grooves in intermediate products having a final sheet thickness
such as cold-rolled sheets.
[0064] As a preferred method for producing an iron core using steel
sheets, for example, there is provided the method including
shearing steel sheets and laminating the sheets without subjecting
them to stress relief annealing. At this time, degradation of iron
loss of the steel sheet due to the shearing can be suppressed to
0.1 W/kg or less (preferably, 0.041 W/kg or less). The production
method is particularly advantageous for producing large iron cores,
for example, in the cases where a steel sheet is sheared into
sheets having a longest side more than 500 mm. Matters including
the number of steel sheets stacked, the size and shape of steel
sheets obtained by the shearing, the presence or absence of the
grooves, the size of the grooves, the presence or absence of
coating, and the type of coating may be appropriately determined on
the basis of ordinary knowledge.
[0065] We found that a small content of an element such as Nb can
considerably reduce degradation of iron loss due to shearing.
[0066] Hereinafter, experiments will be described.
Experiment 1
[0067] Grain oriented electrical steel sheets containing, by mass
%, 3.30% to 3.34% of Si, 0.06% to 0.07% of Mn, 0.025% to 0.028% of
Sb, and 0.03% to 0.04% of Cr; Nb added in various amounts of 4 ppm
(on the level of unavoidable impurities), 22 ppm, 48 ppm, 65 ppm,
90 ppm, and 210 ppm; and the balance being Fe and unavoidable
impurities were produced by a standard production method having
recrystallization annealing (primary recrystallization annealing)
and final annealing (purification annealing). In the final
annealing (purification annealing), the steel sheets were heated at
the maximum steel sheet temperature of 1,200.degree. C. to dissolve
the precipitate-forming element (Nb) therein and then cooled at an
average cooling rate of 20.degree. C./hr from 900.degree. C. to
500.degree. C. and cooled to room temperature.
[0068] The thus-obtain grain oriented electrical steel sheets were
cut into so-called "Epstein" specimens having a size of 30
mm.times.280 mm. At this time, two types of specimens were prepared
by a process of slowly cutting the steel sheets with a wire cutter
such that strain was not caused in the steel and by a general
cutting process for grain oriented electrical steel sheets in which
the steel sheets were cut with a shearing machine employing an
upper blade and a lower blade as described above. The resultant
samples were measured in terms of iron loss in accordance with a
method described in JIS C 2550.
[0069] FIG. 1 shows the results of a study about the relationship
between .DELTA.W (ordinate axis: W/kg) and Nb content in steel
(abscissa axis: mass ppm), .DELTA.W (hereafter, same definition)
being determined by subtracting the iron loss value of a sample
obtained by cutting with the wire cutter from the iron loss value
of a sample obtained by cutting with the shearing machine.
[0070] In the case of cutting with the shearing machine, as
described above, strain remained in the steel sheets and the iron
loss of the steel sheets was degraded. In contrast, cutting with
the wire cutter took a long time, but the steel sheets were cut
substantially without causing strain to remain in the steel
sheets.
[0071] Accordingly, it is believed that .DELTA.W in the figure
substantially represents an iron loss amount equivalent to
degradation due to remaining strain. FIG. 1 thus shows that the
presence of Nb results in reduction of degradation of the iron loss
amount due to shearing.
[0072] The reason why degradation of the iron loss of the
Nb-containing samples was reduced as described above is not
necessarily clear. We believe the following: [0073] An analysis of
the microstructure of the Nb-containing material used in the
experiment revealed that Nb forms precipitates and is dispersed in
the steel. Small precipitates had a diameter of about 0.02 .mu.m
and large precipitates had a diameter of about 3 .mu.m. Since
normal grain oriented electrical steel sheets do not substantially
have such precipitates in steel, we believe that the presence of
the precipitates probably contributed to reduction of degradation
of iron loss due to shearing. [0074] Degradation of iron loss due
to shearing is caused by accumulation of strain in portions having
been subjected to shearing. Accumulation of strain is a phenomenon
where iron atoms regularly arranged in iron crystal grains are
subjected to an external stress or the like and the arrangement of
iron atoms is distorted or becomes irregular.
[0075] Consider a case where the above-described precipitates are
present in such regularly arranged iron atoms. When a stress due to
shearing or the like is applied to a portion containing the
precipitates to cut the portion, the stress is concentrated on the
periphery of the precipitates and cracking is probably generated
before the arrangement of iron atoms is distorted. When it is
considered that such a mechanism relieves the accumulation of
strain, the above-described phenomenon can be explained.
[0076] Although Nb contained in a steel sheet is in two states of
forming a solid solution and forming precipitates, as described
above, it is probably important that Nb forms precipitates. Thus,
the sample containing 22 ppm of Nb was measured in terms of Nb
precipitation percentage (percentage of Nb content in precipitates
with respect to the total Nb content).
[0077] To determine the Nb precipitation (i.e., Nb of Nb
precipitates) percentage, the total Nb content (content in a steel
sheet: mass %) needs to be first determined. The total Nb content
can be determined by inductively-coupled plasma optical emission
spectrometry (ICP optical emission spectrometry) described in JIS G
1237. Note that the contents of Ta, V and Zr can be respectively
determined by methods described in JIS G 1236, JIS G 1221 and JIS G
1232.
[0078] The Nb content in precipitates (content in a steel sheet:
mass %) can be determined by melting a steel sheet by electrolysis
to capture precipitates only (by filtration), measuring the weight
of Nb in the precipitates, and calculating from a decrease in the
weight of the steel sheet due to electrolysis and the weight of Nb
in the precipitates.
[0079] Specifically, the quantitative value of Nb content in
precipitates is determined in the following manner.
[0080] A product sheet is first cur to a size of 50 mm.times.20 mm
and immersed for 2 minutes in a 10% aqueous solution of HCl heated
at 85.degree. C. to remove the coating and film of the product.
After that, the weight of the product sheet is measured. The
product sheet is electrolyzed with a commercially available
electrolytic solution (10% AA solution: 10% acetylacetone-1%
tetramethylammonium chloride-methanol) such that about 1 g of the
product sheet is electrolyzed. To remove precipitates adhering to
the surfaces of the product sheet electrolyzed, the product sheet
is immersed in an ethanol solution and subjected to ultrasonic
waves.
[0081] This ethanol solution and the electrolytic solution used in
the electrolysis, which contain precipitates, are filtrated through
a 0.1 .mu.m-mesh filter paper (allowing capture of minimum
precipitates having a size on the order of nanometers) to capture
the precipitates. After filtration, the precipitates collected by
the filtration are placed together with the filter paper in a
platinum crucible, heated at 700.degree. C. for an hour, mixed with
Na.sub.2B.sub.4O.sub.7 and NaCO.sub.3, and heated at 900.degree. C.
for 15 minutes. The resultant substance is cooled and then heated
at 1,000.degree. C. for 15 minutes.
[0082] After cooling, the substance in the crucible coagulates. The
crucible containing the substance is placed into a 25% aqueous
solution of HCl and the solution containing the crucible is heated
at 90.degree. C. for 30 minutes to melt the entirety of the
substance. The resultant solution is analyzed by ICP optical
emission spectrometry described in JIS G 1237 to determine the
weight of Nb in the precipitates.
[0083] The weight of Nb is divided by a decrease in the weight of
the product sheet (steel sheet) due to electrolysis to determine
the Nb content (mass %) in the precipitates.
[0084] The thus-determined Nb content (mass %) in the precipitates
is divided by the total Nb content (mass %) to determine the Nb
precipitation percentage.
[0085] The Nb precipitation percentage in the sample was 65%. We
further performed studies and have found that precipitation of at
least 10% of the total Nb content is necessary to provide desired
advantages.
[0086] In view of the above-described mechanism, the more the
amount of a precipitate-forming element such as Nb remaining in
steel, the better the .DELTA.W characteristic seems to become.
However, precipitates also degrade the iron loss characteristic of
a material itself to be processed. Accordingly, the amount of
precipitates is preferably small within a range in which
degradation of iron loss due to shearing is small. In Experiment 1,
in materials having a Nb content of 65 ppm or more, the iron loss
of the materials themselves degraded. Hence, the content needs to
be suppressed to 50 ppm or less.
[0087] Next, influence of the crystal grain size of secondary
recrystallized grains on .DELTA.W was studied. This is because we
believe that the presence of a large number of grain boundaries
also probably relieves the accumulation of strain due to shearing.
Accordingly, when the crystal grain size is small and a large
number of grain boundaries are present, there may be cases where
degradation of iron loss due to shearing is naturally small and the
above-described mechanism of relieving accumulation of strain due
to precipitates does not provide advantages.
Experiment 2
[0088] Steel slabs containing, by mass %, 0.035% of C, 3.31% of Si,
0.13% of Mn, 0.039% of Sb, 0.05% of Cr, and 0.012% of P; 42 ppm of
N and 31 ppm of S; and the balance being Fe and unavoidable
impurities were produced by continuous casting, subjected to slab
reheating at 1,250.degree. C., then hot-rolled to provide
hot-rolled sheets having a thickness of 2.7 mm. The hot-rolled
sheets were subsequently annealed at 1,000.degree. C. for 15
seconds and then cold-rolled to provide sheets having a thickness
of 0.30 mm.
[0089] The sheets were subjected to recrystallization annealing in
a 50% N.sub.2-50% H.sub.2 wet atmosphere (decarburization
atmosphere) under soaking conditions in a temperature range of
800.degree. C. to 880.degree. C. for 60 seconds. The sheets were
then coated with an annealing separator mainly containing MgO and
subsequently subjected to purification annealing by being retained
in a temperature range of 1,050.degree. C. to 1,230.degree. C. for
10 hours.
[0090] The temperatures in the recrystallization annealing and the
purification annealing were varied to vary crystal grain size
provided by secondary recrystallization caused in the purification
annealing.
[0091] Flattening annealing that also allows for formation of a
tension coating mainly containing magnesium phosphate and boric
acid was then performed at 900.degree. C. for 15 seconds. The
resultant sheets were cut to have the size of Epstein specimens (30
mm.times.280 mm). At this time, as in Experiment 1, cutting with a
wire cutter and cutting with a shearing machine were performed. The
resultant samples were measured in terms of iron loss in accordance
with a method described in JIS C 2550.
[0092] After that, the steel substrates were exposed by pickling
and the crystal grain size of secondary recrystallized grains was
measured. For each condition, the crystal grain size was determined
by measuring the grain sizes of four Epstein specimens and
averaging the measured grain sizes. Analysis of the components of
the steel substrates revealed 0.0018% of C, 3.30% of Si, 0.13% of
Mn, 0.039% of Sb, 0.05% of Cr, and 0.011% of P, and the contents of
the other elements were less than the detection limits. The
relationship between .DELTA.W (ordinate axis: W/kg) determined in
the above-described manner and crystal grain size (abscissa axis:
mm) is illustrated in FIG. 2.
[0093] In Experiment 2, since precipitate-forming elements such as
Nb did not remain, the advantages provided in Experiment 1 were not
exhibited. Accordingly, when the average grain size was large,
.DELTA.W was large; when the average grain size was small, .DELTA.W
was small. Stated another way, the effect of decreasing .DELTA.W by
addition of a precipitate-forming element such as Nb is exhibited
when the average grain size of secondary recrystallized grains is 5
mm or more.
[0094] From the above-described experiments, we found that, by
making a final product sheet of a grain oriented electrical steel
sheet having a large grain size of secondary recrystallized grains
contain 10 to 50 ppm of an element such as Nb and by making at
least 10% of the content of the element be present in the form of
precipitates, degradation of iron loss due to shearing can be
suppressed.
EXAMPLE 1
[0095] Steel slabs containing 0.065% of C, 3.25% of Si, 0.13% of
Mn, 240 ppm of Al, 70 ppm of N, 36 ppm of S, and 25 ppm of Nb (for
No. 7 steel only, 20 ppm of Nb), and the balance being Fe and
unavoidable impurities, were produced by continuous casting. The
steel slabs were subjected to slab reheating at 1,400.degree. C.
and then hot-rolled to sheets to a thickness of 2.4 mm. The sheets
were then subjected to hot-rolled sheet annealing at 1,000.degree.
C. for 40 seconds, subsequently to cold rolling so as to have a
thickness of 1.6 mm, to intermediate annealing at 900.degree. C.,
and then to cold rolling to sheets so as to have a thickness of
0.23 mm.
[0096] The resultant sheets were then subjected to
recrystallization annealing in a 60% N.sub.2-40% H.sub.2 wet
atmosphere under soaking conditions at 850.degree. C. for 90
seconds, subsequently coated with an annealing separator mainly
containing MgO, and subjected to purification annealing at
1,220.degree. C. for 6 hours. In the purification annealing, the
cooling rate for a range of 900.degree. C. to 500.degree. C. was
controlled as described in Table 1 to thereby vary the diameter of
Nb precipitates and Nb precipitation percentage. After that, the
sheets were subjected to flattening annealing at 850.degree. C. for
20 seconds.
[0097] The obtained samples were cut to a size of 30 mm.times.280
mm. At this time, the cutting was performed under two conditions:
cutting with a wire cutter and cutting with a shearing machine.
Magnetic characteristics of obtained samples were measured by the
method described in JIS C 2550 and the magnetic characteristics of
the samples obtained by the cutting with the wire cutter are
described in Table 1.
[0098] As for iron losses in terms of the two cutting-process
conditions, .DELTA.W determined by subtracting the iron loss of a
sample obtained by cutting with the wire cutter from the iron loss
of a sample obtained by cutting with the shearing machine is also
described in Table 1.
[0099] The samples having been subjected to the magnetic
measurement were then subjected to pickling to remove coatings and
the crystal grain size of secondary recrystallized grains was
measured. The results are also described in Table 1 together with
the measurement results of the diameter and precipitation
percentage of Nb precipitates. After pickling, the component
composition of steel sheets of the coating-removed samples was
measured. As a result, the component composition confirmed was
0.0016% of C, 3.24% of Si, 0.13% of Mn, and 18 ppm of Nb (for No. 7
steel only, 15 ppm of Nb), which satisfied our requirements.
TABLE-US-00001 TABLE 1 Cooling Crystal Precipitate Precipitation
rate grain size diameter percentage B.sub.8 W.sub.17/15 .DELTA.W
No. (.degree. C./hr) (mm) (.mu.m) (%) (T) (W/kg) (W/kg) Remark 1
2.2 19.8 4.5 98 1.946 0.811 0.101 Comparative Example 2 5.5 20.1
2.5 94 1.942 0.823 0.061 Example 3 7.8 21.2 0.78 91 1.942 0.826
0.036 Example 4 14.0 20.5 0.35 68 1.938 0.795 0.022 Example 5 30
20.2 0.12 55 1.945 0.825 0.038 Example 6 100 17.4 0.08 34 1.938
0.799 0.041 Example 7* 100 18.5 0.08 14 1.94 0.81 0.055 Example 8
250 22 0.03 7 1.937 0.846 0.113 Comparative Example 9 2400 22.6
0.01 3 1.921 1.108 0.214 Comparative Example *The Nb content was 20
ppm in the slab and 15 ppm in the coating-removed sample.
[0100] As described in Table 1, all of our Examples in which the
crystal grain size and the diameter and precipitation percentage of
Nb precipitates satisfy appropriate ranges have good magnetic
characteristics and small .DELTA.W, which shows that degradation of
iron loss due to shearing is small.
EXAMPLE 2
[0101] Product sheets (sheet thickness: 0.23 mm) of grain oriented
electrical steel sheets were provided that contained components
described in Table 2 and that were produced by a standard
production method in which recrystallization annealing was
performed, followed by purification annealing at 1,150.degree. C.,
and cooling at a cooling rate in the range of 900.degree. C. to
500.degree. C. of 25.degree. C./hr.
[0102] The grain oriented electrical steel sheets were cut to a
size of 30 mm.times.280 mm. At this time, the cutting was performed
under two conditions: cutting with a wire cutter and cutting with a
shearing machine.
[0103] The magnetic characteristics of the obtained samples were
measured by the method described in JIS C 2550 and the magnetic
characteristics of the samples obtained by the cutting with the
wire cutter are described in Table 2. In addition, .DELTA.W
determined as in EXAMPLE 1 is also described in Table 2.
[0104] The samples having been subjected to the magnetic
measurement were subjected to pickling to remove coatings and the
crystal grain size of secondary recrystallized grains was measured.
The results are also described in Table 2 together with the
measurement results of the diameter and precipitation percentage of
precipitates of Nb or the like. Note that the component
compositions of steel sheets in Table 2 are results obtained by
measuring the component compositions of coating-removed samples
after the pickling.
[0105] In addition, the precipitates were measured. As a result,
the precipitates had an average diameter of 0.05 to 3.34 .mu.m and
a precipitation percentage of 0% to 79%.
TABLE-US-00002 TABLE 2 C Si Mn Ni Cr Cu P Sn Sb Bi Mo No. (%) (%)
(%) (%) (%) (%) (%) (%) (%) (%) (%) Other 7 0.0011 3.11 0.02 -- --
-- 0.005 -- -- -- -- Ta: 30 ppm 8 0.0026 3.12 0.08 -- 0.03 0.01 --
0.045 -- -- -- Ta: 40 ppm 9 0.0031 2.55 0.32 0.06 0.12 0.03 0.02 --
0.065 -- -- Ta: 30 ppm 10 0.0020 3.31 0.50 -- 0.06 -- 0.01 -- --
0.015 -- Ta: 30 ppm 11 0.0017 3.20 0.09 -- -- -- 0.008 -- -- --
0.013 Ta: 20 ppm 12 0.0022 3.35 0.09 -- 0.05 0.01 0.012 -- 0.033 --
0.011 Ta: 80 ppm 13 0.0026 3.13 0.07 -- 0.06 -- 0.011 0.078 0.023
-- -- Nb: 40 ppm 14 0.0025 3.30 0.13 -- 0.05 -- 0.011 -- 0.123 --
-- V: 30 ppm 15 0.0019 3.22 0.12 -- 0.07 -- 0.013 -- 0.056 -- --
Zr: 20 ppm 16 0.0020 3.25 0.63 -- 0.05 -- 0.021 -- 0.058 -- -- Nb:
18 ppm, Ta: 20 ppm 17 0.0032 2.98 0.07 -- 0.05 -- 0.018 -- 0.04 --
-- V: 20 ppm, Zr: 20 ppm, Nb: 9 ppm 18 0.0017 3.01 0.21 -- 0.06 --
0.018 -- 0.052 -- -- -- 19 0.0028 3.34 0.17 -- 0.05 -- 0.015 --
0.039 -- -- Nb: 35 ppm 20 0.0015 3.21 0.06 -- -- -- -- -- -- -- --
Ta: 30 ppm 21 0.0020 3.25 0.06 -- -- -- -- -- -- -- -- Nb: 40 ppm
22 0.0020 3.11 0.15 -- -- -- -- -- -- -- -- V: 30 ppm 23 0.0031
3.35 0.20 -- -- -- -- -- -- -- -- Zr: 30 ppm Precipitate
Precipitation Crystal diameter percentage grain size B.sub.8
W.sub.17/50 .DELTA.W No. (.mu.m) (%) (mm) (T) (W/kg) (W/kg) Remark
7 0.34 42 12.3 1.943 0.798 0.031 Example 8 0.08 38 7.8 1.955 0.760
0.022 Example 9 0.15 41 16.5 1.942 0.821 0.015 Example 10 0.08 55
10 1.938 0.839 0.038 Example 11 0.52 60 12.1 1.937 0.824 0.021
Example 12 3.34 79 10.5 1.940 1.135 0.026 Comparative Example 13
0.35 50 23.9 1.966 0.759 0.025 Example 14 0.19 43 17.7 1.956 0.780
0.034 Example 15 0.05 31 14.4 1.942 0.803 0.025 Example 16 0.07 32
20.5 1.946 0.845 0.041 Example 17 0.20 40 13.4 1.950 0.847 0.036
Example 18 -- 0 22.0 1.953 0.795 0.186 Comparative Example 19 0.51
35 2.3 1.884 1.035 0.023 Comparative Example 20 0.08 40 20.1 1.950
0.791 0.023 Example 21 0.07 42 25.4 1.960 0.777 0.040 Example 22
0.08 36 18.0 1.942 0.801 0.035 Example 23 0.45 51 17.2 1.943 0.809
0.039 Example
[0106] As described in Table 2, all of our Examples in which the
crystal grain size and the diameter and precipitation percentage of
precipitates of Nb or the like satisfy appropriate ranges have good
magnetic characteristics and small .DELTA.W, which shows that
degradation of iron loss due to shearing is small.
EXAMPLE 3
[0107] Steel slabs containing 0.065% of C, 3.25% of Si, 0.13% of
Mn, 0.05% of Cr, 240 ppm of Al, 70 ppm of N, 36 ppm of S, 0.013% of
P, 0.075% of Sn, 0.036% of Sb, 0.011% of Mo, and 25 ppm of Nb, and
the balance being Fe and unavoidable impurities, were produced by
continuous casting. The steel slabs were subjected to slab
reheating at 1,400.degree. C. and then hot-rolled to sheets to a
thickness of 2.4 mm. The sheets were then subjected to hot-rolled
sheet annealing at 1,000.degree. C. for 40 seconds, subsequently to
cold rolling so as to have a thickness of 1.6 mm, to intermediate
annealing in a temperature range of 700.degree. C. to 1,020.degree.
C., and then to cold rolling to provide steel sheets having a
thickness of 0.23 mm.
[0108] Linear grooves having a width of 100 .mu.m and a depth of 25
.mu.m were then formed by local electrolytic etching in the
surfaces of the steel sheets to extend at an angle of 10.degree.
with respect to a direction perpendicular to the rolling direction
at a pitch of 8 mm. The sheets were then subjected to
recrystallization annealing in a 60% N.sub.2-40% H.sub.2 wet
atmosphere under soaking conditions at 800.degree. C. to
900.degree. C. for 90 seconds. The sheets were then coated with an
annealing separator mainly containing MgO and subsequently
subjected to purification annealing at 1,220.degree. C. for 6
hours. After that, the sheets were cooled such that they were
cooled from 900.degree. C. to 500.degree. C. at a cooling rate of
10.degree. C./hr.
[0109] The sheets were then subjected to flattening annealing at
850.degree. C. for 20 seconds. The temperatures of the intermediate
annealing and the temperatures of the recrystallization annealing
were varied to vary the grain size after secondary
recrystallization. The obtained samples were cut into Epstein
specimens having a size of 30 mm.times.280 mm. At this time, the
cutting was performed under two conditions: cutting with a wire
cutter and cutting with a shearing machine.
[0110] The magnetic characteristics of the obtained samples were
measured by the method described in JIS C 2550 and the magnetic
characteristics of the samples obtained by the cutting with the
wire cutter are described in Table 3. In addition, .DELTA.W
determined as in EXAMPLE 1 is also described in Table 3.
[0111] The samples having been subjected to the magnetic
measurement were subjected to pickling to remove coatings and the
crystal grain size of secondary recrystallized grains was measured.
The results are also described in Table 3 together with the
measurement results of the diameter and precipitation percentage of
Nb precipitates. After pickling, the component composition of steel
sheets of the coating-removed samples was measured. As a result,
the component composition confirmed was 0.0016% of C, 3.24% of Si,
0.13% of Mn, 0.05% of Cr, 0.011% of P, 0.074% of Sn, 0.036% of Sb,
0.011% of Mo, and 18 ppm of Nb, which satisfied our
requirements.
TABLE-US-00003 TABLE 3 Crystal grain Precipitate Precipitation
B.sub.8 W.sub.17/50 .DELTA.W No. size (mm) diameter (.mu.m)
percentage (%) (T) (W/kg) (W/kg) Remark 24 3.8 0.56 71 1.855 0.867
0.015 Comparative Example 25 7.5 0.78 65 1.908 0.695 0.024 Example
26 12.6 0.47 52 1.915 0.690 0.026 Example 27 15.0 0.33 48 1.910
0.681 0.033 Example 28 21.9 0.41 60 1.922 0.689 0.034 Example 29
25.2 0.59 72 1.918 0.677 0.030 Example
[0112] As described in Table 3, all of our Examples in which the
crystal grain size and the diameter and precipitation percentage of
Nb precipitates satisfy appropriate ranges have good magnetic
characteristics and small .DELTA.W, which shows that degradation of
iron loss due to shearing is small.
[0113] EXAMPLES 1 to 3 show that grain oriented electrical steel
sheets substantially having a .DELTA.W of 0.1 W/kg or less and
undergoing little degradation of magnetic characteristics due to
shearing can be provided. Accordingly, production of a laminated
iron core by shearing a steel sheet without performing stress
relief annealing is effective for enhancing the magnetic
characteristics of the iron core, in particular, for achieving
improvement in terms of iron loss.
[0114] In particular, in the steels containing Nb precipitates in
EXAMPLES 1 to 3, the diameter (average diameter) of the
precipitates is 0.12 .mu.m or more and 1.2 .mu.m or less
(preferably 0.78 .mu.m or less; the precipitation percentage is
preferably 48% or more) and .DELTA.W is 0.038 W/kg or less. Thus,
better characteristics can be achieved. EXAMPLES 1 to 3 and the
like show that, to achieve the diameter and amount of precipitates,
the cooling rate after final annealing is preferably made
7.8.degree. C./hr to 30.degree. C./hr, more preferably 7.8.degree.
C./hr to 14.degree. C./hr.
INDUSTRIAL APPLICABILITY
[0115] Degradation of magnetic characteristics of a grain oriented
electrical steel sheet due to shearing can be reduced. As a result,
iron cores having a low iron loss can be obtained and thus, for
example, large transformers having high energy efficiency can be
produced.
* * * * *